2. aimed to modify the properties of TiO2, specifically to decrease
the electron−hole recombination rate18
while extending its
light absorption into the visible region. Previously, we examined
the synthesis of titania−graphene composites21
and iron-doped
titania nanoassemblies in the green solvent, supercritical CO2,22
as well as their resulting properties, which improved
significantly compared to bare titania. Our group has also
carried out DFT simulations on the behavior of titania in CO2,
demonstrating CO2-philicity arising from the metal acetate
groups,23,24
illustrating the importance of theoretical calcu-
lations for understanding the physical and chemical mecha-
nisms operating in these systems.
To date, a detailed theoretical understanding of the chemical
and physical interactions between TiO2 nanostructures and
SWCNTs as well as their charge transfer mechanism is
unknown. DFT calculations,12
studying the photovoltaic
properties of interfaces of bulk titanium with mixed semi-
conducting and metallic CNTs, have shown that TiO2/CNT
interfaces can be useful as photovoltaic materials if they are
decorated by a metal cluster. However, the details on the
chemical and electronic interactions between nanostructured
TiO2 and pure or functionalized CNTs have not been
investigated theoretically. In the present study, we investigated
the interaction of TiO2 and CNTs through two possible
adsorption mechanisms: physisorption and chemisorption
along with detailed charge transfer calculations.
■ COMPUTATIONAL DETAILS
Electronic structure calculations were carried out using the
GGA PW9125
functional implemented in VASP code26,27
for all
CNT and titania systems. The GGA PW91 functional
previously provided a higher efficiency for stabilizing the
anionic adsorption of carbon-based compounds to TiO2
surfaces.28
The electron−ion interaction is described by the
projector-augmented wave (PAW) scheme,29,30
the electronic
wave functions were expanded using plane waves with a kinetic
energy up to 400 eV, and the k-point sampling was set to 3 × 2
× 1 for the geometry optimization of periodic systems
specifically and to 5 × 5 × 1 for the electronic structure. The
Brillouin zone was described using a Monkhorst−Pack31
(M&P) scheme of special k-points. Convergence criteria of 5
× 10−3
eV for energies and 0.01 eV/Å for forces acting on ions
in structural optimizations were used. Band diagrams and
density of states (DOS) analysis were obtained by fixing the
Wigner−Seitz radius (rwigs) for the support during integration
over the number of electrons and then by setting rwigs for the
adsorbates within the radii of tangential spheres. This method
allowed the accurate assignment of relevant atomic orbital
attributions to a particular projected DOS peak. All systems
were modeled using the supercell approach with periodically
repeated slabs. Models of pure and functionalized armchair and
zigzag CNT substrates were used. Six adsorption sites were
considered: top, bridge, and hollow sites on the pure CNT and
CNT-ol, carboxylate, and epoxy sites on the functionalized
CNT. Parts a−c of Figure 1 show the schematic structures of
titania species adsorbed on all possible sites of a pure CNT,
while parts d−f of Figure 1 show the potential adsorption sites
for titania species on functionalized CNTs.
Single walled carbon nanotubes (SWCNTs) were con-
structed by rolling up graphene to form a cylinder. The
circumference of the SWCNT is determined by the two
primitive vectors ⎯→a1 and ⎯→a2 , the chiral vector Ch = n⎯→a1 + m⎯→a2 ,
and the lattice parameter of the graphene honeycomb structure
a0. The primitive vectors of graphene are the following
⎯→ = ⎯→ = −
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟a aa a3
1
2
,
3
2
, 3
1
2
,
3
2
1 20 0
(1)
where a0 = 1.42 Å is the C−C bond length. The 2D graphene
sheet together with the ⎯→a1, ⎯→a2 , and Ch vectors specifying the
chirality of the nanotube are as shown in Figure 2. A lattice
point O is chosen as the origin.
Figure 2 also shows the physical properties of the carbon
nanotubes formed with respect to the pair of integers (n, m).3,32
Both metallic and semiconducting SWCNTs can be formed
from armchair, zigzag, and chiral tubules.33
The diameter of a
(5,5) armchair SWCNT is expected to be slightly longer than
the diameter of a (8,0) zigzag SWCNT, respectively 6.78 and
6.27 Å theoretically. A (9,0) zigzag SWCNT is closer in
diameter, 7.05 Å, to a (5,5) armchair SWCNT but has similar
physical properties, as it is also metallic. A semiconducting
(8,0) zigzag SWCNT is preferred as opposed to the metallic
properties of the (5,5) armchair SWCNT.3,32
Figure 1. Adsorption sites of clean CNT and functionalized CNTs.
TiO2 adsorbates can be located at (a) top, (b) bridge, and (c) hollow
sites of CNT. TiO2 can also be located at (d) carboxylate, (e) CNT-ol,
and (f) epoxy sites of functionalized CNTs.
Figure 2. 2D graphene sheet shown with integers (n, m) specifying
chiral vectors Ch for carbon nanotubes, including zigzag if n or m
equals zero or armchair if n = m. The red circled dots denote metallic
tubules, while the small green dots are for semiconducting tubules.3,32
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3. Chemical functionalization is known experimentally to occur
preferentially at the edges of SWCNTs.7,8,10,11
As the
limitations of DFT calculations allow only small system sizes,
functionalized CNTs were used separately from pure SWCNTs
to study the adsorption on newly formed adsorption sites due
to functionalization. The supercells for pure and functionalized
CNTs are shown in Figure 3, which contain 160 carbon atoms.
Pure armchair and zigzag CNTs were fully optimized in
rectangular simulation boxes of respectively 20.0 Å × 15.0 Å ×
30.0 Å and 21.1 Å × 15.0 Å × 30.0 Å, as shown on Figure 3a
and c. The finite and functionalized armchair and zigzag
SWCNTs were also fully optimized in rectangular simulation
boxes of 25.0 Å × 15.0 Å × 30.0 Å, as shown in Figure 3b and
d. Spin polarization was considered in all calculations, and the
electronic structures were optimized to their ground states.
Figure 4a and b show two of the most stable titania surfaces,
namely, the (100) and (001) for rutile and the (010) and (100)
for anatase.13,14
To provide an accurate description of a site by
site adsorption to SWCNTs, smaller models of titania were
considered. Rigid structures were set up for both rutile and
anatase due to the previously reported metastability of small
size titania nanostructures of bulk properties.34
Models of a
TiO2 molecule converged in a vacuum, a rutile nanostructure
(Ti2O4), and an anatase nanostructure (Ti4O8) were used both
containing the smallest stoichiometric ratios that provided
accurate structure yet facilitated computational convergence.6,35
The anatase unit cell has a more compact structure than rutile,
as the bond distances and angles are slightly smaller.6,21
Figure
4 also shows the schematic structures of the titania slabs
containing stoichiometric models for site by site adsorption.
The isolated TiO2 molecule was calculated in a large
rectangular supercell (10.0 × 10.0 × 20.0 Å) and structurally
optimized. The rutile TiO2 unit cell (Ti2O4) was initially
calculated in a small rectangular supercell (4.6 × 4.6 × 2.9 Å)
for a structural optimization of bulk properties. Then, the
isolated rutile had its wave function optimized in a large
rectangular supercell (10.0 × 10.0 × 20.0 Å). The anatase TiO2
unit cell (Ti4O8) was initially calculated in a small rectangular
supercell (3.7 × 3.7 × 9.5 Å) also for a geometry optimization
in bulk. After that, anatase had its wave function optimized in a
large rectangular supercell (10.0 × 10.0 × 20.0 Å). Spin
polarization was considered in all calculations, and the
electronic structures were optimized to their ground state.
The adsorption energy (Eads) is calculated according to
= − +E E E E( )ads (TiO /CNT) TiO CNT2 2 (2)
where E(CNT), E(TiO2), and E(TiO2/CNT) denote,
respectively, the calculated energy of a pure CNT, the isolated
titanium oxide molecule or nanostructure in a vacuum, and the
total energy of a TiO2/CNT unit cell adsorbed to the CNT. A
negative value of Eads implies that the adsorption of the
crystalline TiO2 adsorbate is thermodynamically stable on its
CNT substrate.
■ RESULTS AND DISCUSSION
Interaction of TiO2 Species with Functionalized
Armchair (5,5) SWCNTs. Armchair SWCNTs have three
main physical adsorption sites on their outside wall,36
which are
the top, bridge, and hollow sites, as described in Figure 1.
Figure 3. Optimized structures of (a) a (5,5) armchair SWCNT, (b) a (5,5) functionalized armchair SWCNT, (c) a (8,0) zigzag SWCNT, and (d) a
(8,0) functionalized zigzag SWCNT.
Figure 4. Optimized structures of (a) bulk rutile titania with the
highlighted Ti2O4 structural unit cell and (b) bulk anatase titania with
the highlighted Ti4O8 structural unit cell. Pink and red spheres
respectively represent titanium and oxygen.35
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4. Functionalized armchair SWCNTs can have additional
adsorption sites, epoxy, CNT-ol, and carboxylate, due to
functional groups that are introduced onto their surface during
synthesis, which are concentrated at the edges. Functionalized
SWCNT edges have been reported as favoring electron
transport while binding to flat titania surfaces.37,38
The
interaction between titania species and armchair (5,5)
SWCNTs was studied on all physical and chemical adsorption
sites summarized in Table 1. The physical adsorption sites
depend on the geometry of the adsorbate and direction of
adsorption. Rotating the adsorbate by 90° created the “rotated”
version of a physical adsorption site.
Figure 5a shows a comparison of the adsorption strength of
the three studied species: molecular TiO2, rutile (Ti2O4), and
anatase (Ti4O8) adsorbed at similar adsorption sites that are the
top, the bridge, and the hollow site of pure CNT. Figure 5b
shows the identical adsorption sites of CNT, with the titania
adsorbates rotated by 90°; these adsorption sites are termed
rotated top, rotated bridge, and rotated hollow. Finally, Figure
5c shows all the Ti−O interactions between the titania species
and the organic adsorption sites that are carboxylate, CNT-ol,
and epoxy.
Molecular TiO2 was found to physisorb preferentially on a
bridge site, followed by the hollow site and then the top site of
armchair SWCNTs. The rotated hollow site was found to be
the most favorable, slightly above 1.6 eV, followed by the
rotated bridge and the rotated top of an adsorbed rotated
molecular TiO2 on CNT. Molecular TiO2 was found to bind
the closest to armchair SWCNT on the top and rotated top
sites at 2.55 and 2.52 Å, respectively. The binding energy and
distances correspond to a Ti−C noncovalently bonded
interaction39,40
in the physical adsorption process. The TiO2
molecule was found to chemisorb preferentially on the edge-
located carboxylate site, slightly above 3.6 eV, followed by the
edge located CNT-ol site. TiO2 adsorbs to the CNT-ol site
closest at a Ti−O noncovalently bonded distance of 1.99 Å.
The epoxy adsorption site of the CNTs was found to have the
weakest binding energy of the Ti−O nonbonded interactions
attributed to the fact that titania physisorbs to epoxy while it
chemisorbs to carboxylate and CNT-ol.
Rutile (Ti2O4) was found to adsorb preferentially on a
hollow site, followed by the bridge site and then the top site of
armchair SWCNT. The rotated hollow site was found to be the
most favorable, slightly above 2.4 eV, followed by the rotated
top and then the rotated bridge of an adsorbed rotated rutile
(Ti2O4) on armchair CNT. Rutile binds the closest to armchair
SWCNT on the top and rotated top sites at 2.21 Å. Rutile
(Ti2O4) adsorbs preferentially on the edge located carboxylate
Table 1. Adsorption Energies (Eads) and Interaction Distances (Dads) for TiO2 Species Adsorbed on Armchair CNTs
molecular TiO2 rutile or Ti2O4 anatase or Ti4O8
site/(interaction type) Eads (eV) Dads (Å) Eads (eV) Dads (Å) Eads (eV) Dads (Å)
top/(Ti−C) −1.38 2.55 −2.07 2.21 −1.65 2.13
bridge/(Ti−C) −1.51 2.58 −2.34 2.28 −1.92 2.13
hollow/(Ti−C) −1.48 2.74 −2.44 2.47 −2.05 2.40
rotated top/(Ti−C) −1.22 2.52 −2.05 2.21 −1.52 2.15
rotated bridge/(Ti−C) −1.44 2.56 −1.76 2.27 −1.23 2.32
rotated hollow/(Ti−C) −1.63 2.72 −2.41 2.45 −3.10 2.37
epoxy/(Ti−O) −1.20 2.25 −1.83 2.03 −1.94 2.05
carboxylate/(Ti−O) −3.63 2.12 −5.79 2.03 −5.41 1.95
CNT-ol/(Ti−O) −2.19 1.99 −3.61 1.90 −3.11 1.89
Figure 5. Adsorption energies of TiO2 species on armchair functionalized CNTs. (a) Adsorption energy per titania species adsorbed on top, bridge,
and hollow. (b) Adsorption energy per titania species adsorbed on rotated top, rotated bridge, and rotated hollow. (c) Adsorption energy per titania
species adsorbed on carboxylate, CNT-ol, and epoxy.
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5. site followed by the edge located CNT-ol site. The carboxylate
site adsorbs rutile (Ti2O4) the strongest, around 5.8 eV, but this
adsorption site can only be found on the edges of the CNT.
The epoxy adsorption site of the CNTs was found to have the
weakest binding energy of the Ti−O nonbonded interactions,
slightly above 1.8 eV.
The rotated hollow was found to be the site onto which
anatase (Ti4O8) adsorbs the strongest on armchair SWCNTs;
the binding energy here is slightly above 3.1 eV. Anatase
physisorbs preferentially to rotated hollow and hollow sites
followed by the other physical adsorption sites including the
epoxy. Anatase (Ti4O8) was found to chemisorb preferentially
on the edge located carboxylate site followed by the edge
located CNT-ol site of the functionalized armchair SWCNT.
The carboxylate site adsorbs anatase (Ti4O8) the strongest,
above 5.4 eV, but this adsorption site can only be found on the
edges of the CNT. The epoxy adsorption site of the CNTs was
found to have the weakest binding energy of the Ti−O
nonbonded interactions around 1.9 eV.
Interaction of TiO2 Species with Functionalized
Zigzag (8,0) SWCNTs. Zigzag SWCNTs also have three
main physical adsorption sites on their outside wall,36
which are
the top, bridge, and hollow sites and additional adsorption sites,
epoxy, CNT-ol, and carboxylate, due to functionalization. The
interactions between titania species and zigzag (8,0) SWCNTs
were studied on all physical and chemical adsorption sites. The
results of DFT calculated noncovalently39,40
bonded Ti−C and
Ti−O binding energies and distances between titania and
zigzag SWCNT are shown in Table 2.
Figure 6a shows a comparison of adsorption strengths of the
three studied species, the molecular TiO2, the rutile (Ti2O4),
and the anatase (Ti4O8), adsorbed at similar adsorption sites
that are the top, the bridge, and the hollow site of pure CNTs.
Figure 6b shows the identical adsorption sites of CNT, with the
similar titania adsorbates which were rotated by 90°. Finally,
Figure 6c shows all the Ti−O interactions between the titania
species and the organic adsorption sites that are carboxylate,
CNT-ol, and epoxy.
A similar trend in Ti−C interactions can be observed, while
the titania species adsorb on zigzag SWCNTs compared to
adsorption on armchair SWCNTs. Some differences on
physical adsorption sites can still be observed. For molecular
TiO2, the bridge site was found to be the most favorable
energetically, slightly below 1.57 eV. Rutile (Ti2O4) was found
to show a higher binding strength on the hollow site, slightly
above 2.8 eV, while anatase (Ti4O8) showed the highest overall
physical adsorption energy on the hollow site, slightly above 3.1
eV. For all titania species, the Ti−O interaction adsorption sites
Table 2. Adsorption Energies (Eads) and Interaction Distances (Dads) for TiO2 Species Adsorbed on Zigzag CNTs
molecular TiO2 rutile or Ti2O4 anatase or Ti4O8
site/(interaction type) Eads (eV) Dads (Å) Eads (eV) Dads (Å) Eads (eV) Dads (Å)
top/(Ti−C) −1.45 2.46 −2.33 2.25 −1.90 2.13
bridge/(Ti−C) −1.57 2.50 −2.41 2.34 −2.12 2.12
hollow/(Ti−C) −1.52 2.84 −2.86 2.47 −3.11 2.40
rotated top/(Ti−C) −1.42 2.48 −2.44 2.25 −2.04 2.15
rotated bridge/(Ti−C) −1.53 2.52 −2.39 2.37 −1.94 2.30
rotated hollow/(Ti−C) −1.40 2.84 −2.71 2.53 −2.56 2.41
epoxy/(Ti−O) −1.15 2.20 −1.63 2.10 −0.83 2.11
carboxylate/(Ti−O) −3.83 2.10 −5.48 2.00 −5.27 1.97
CNT-ol/(Ti−O) −1.71 2.04 −2.68 1.92 −2.11 1.89
Figure 6. Adsorption energies of TiO2 species on zigzag functionalized CNTs. (a) Adsorption energy per titania species adsorbed on top, bridge, and
hollow. (b) Adsorption energy per titania species adsorbed on rotated top, rotated bridge, and rotated hollow. (c) Adsorption energy per titania
species adsorbed on carboxylate, CNT-ol, and epoxy.
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6. showed similar trends, as carboxylate remains the strongest,
followed by the CNT-ol and then the epoxy site.
Analysis of the Interaction between TiO2 and
SWCNTs. The binding energy values of the TiO2 species
adsorbed on SWCNTs, reported in Tables 1 and 2, show a
quantitative difference between solid state TiO2 and gas phase
TiO2 on all adsorption sites. When adsorbed on CNTs, solid
state TiO2 is thermodynamically more stable than gas phase
TiO2 due to a lower entropy of adsorption.41
As a result,
binding distances of structural TiO2 are shorter than molecular
TiO2 for similar reasons. The calculated strength and distances
of adsorption on functionalized SWCNT sites confirm this
tendency. Rutile TiO2 generally binds stronger while anatase
binds closer to SWCNTs. Both rutile and anatase bind stronger
and closer to SWCNTs than molecular TiO2. There are two
adsorption sites on armchair and zigzag SWCNTs that show a
higher binding strength for anatase TiO2 with respect to rutile.
These sites are the rotated hollow site of armchair (5,5)
SWCNT and the hollow site of zigzag (8,0) SWCNT.
Further understanding on the difference of binding energies
for TiO2 species adsorbed to the SWCNTs can be gained by
calculating the real space charge redistribution, as the binding
energy may have electrostatic origins.
Figure 7a−d show the electrostatic density map at the
binding region of Ti2O4 adsorbed on, respectively, hollow,
epoxy, carboxylate, and CNT-ol sites of zigzag SWCNTs. The
Ti−C interaction of Ti2O4 with a hollow site of zigzag CNT is
perpendicular to the CNT surface along the tube axis, and the
electrostatic Ti−O interaction of Ti2O4 with epoxy, carboxylate,
or CNT-ol is parallel to the CNT surface along the tube axis.
Most of the rutile electronic density is created by the presence
of oxygen. The density maps in Figure 7a and b show that a
clear electrostatic gap exists in the binding region as titania
physisorbs to both hollow and epoxy sites. The proximity of
rutile to the six carbons of the SWCNT hollow site creates an
additional binding strength helping to explain the difference
between the adsorption on hollow with the adsorption on other
physical adsorption sites.21
The density contour maps shown in
Figure 7c and d indicate that a much larger electrostatic
interaction exists between the substrate terminated by
(COO−) and also (O−), and the titania. This is attributed
to the fact that in these cases titania chemisorbs to carboxylate
and CNT-ol. A clear concentration of charge renders a
continuous electron density along the z axis. The electrostatic
density of titania−(COO−) and titania−(O−) binding regions
confirms the presence of a maximum force of attraction due to
the ionic nature of carboxylate and CNT-ol adsorption sites.
For the carboxylate group, the charge density is increasingly
accumulated along the nonbonded interaction axis of the
TiO2−CNT binding region due to the presence of the two
oxygen atoms. These maps confirm the importance of the
electron distribution property in the interaction between titania
and SWCNTs.
Although the electronic structures of rutile and anatase TiO2
were previously investigated18,42
along with CNTs5,43
and
functionalized CNTs,7−11,44
the electronic structure of TiO2 on
CNT has only been considered for larger diameter CNTs
having interfacial interactions with bulk TiO2.12
Hence, we
investigated the partial density of states (PDOS) of anatase
TiO2 nanostructures adsorbed on armchair SWCNTs at the
calculated most stable adsorption sites, i.e., the hollow site and
the rotated hollow site. After the binding energetics and
structural morphologies of TiO2 clusters adsorbed on SWCNTs
and functionalized SWCNTs were investigated, the electronic
structures of all systems were studied in order to determine the
relation between PDOS and the binding energy. As the
difference between rutile and anatase titania has already been
studied by PDOS,18,42
we focused our study on the difference
in binding strength for anatase TiO2 adsorbed on hollow and
rotated hollow sites of armchair and zigzag SWCNTs. As
previously mentioned, rotating the anatase TiO2 nanostructures
adsorbed on CNTs by 90° from hollow to rotated hollow
increases the binding energy by more than 1.1 eV for the
armchair CNTs (see Table 1) and decreases the binding energy
by 0.6 eV for the zigzag CNTs. The PDOS for the C 2p band of
CNT and the O 2p and Ti 3d bands of anatase TiO2 adsorbed
on a hollow site and a rotated hollow site of armchair SWCNT
are displayed, respectively, in Figure 8a and b. To facilitate an
understanding of how O 2p, Ti 3d, and C 2p states are
modified upon adsorption, the C 2p band of armchair SWCNT
was isolated from that of TiO2 after adsorption of nano-
structural TiO2.
While adsorbed on hollow and rotated hollow sites of
armchair SWCNTs, the PDOS of O and Ti atoms of anatase
titania display clear semiconducting properties. Their valence
and conduction bands are spaced around the Fermi level (EF)
referenced at 0 eV. The valence band is dominated by O 2p
orbitals with a small contribution from the Ti 3d orbitals,
whereas Ti 3d dominates the conduction band with a small
contribution from O 2p. The intrinsic band gaps of TiO2
nanostructures have no changes, implying that the electron
transition from the O 2p at the valence band and the Ti 3d at
the conduction band is not the dominant process in the
interactions between anatase TiO2 and armchair CNT. The
lower theoretical value of band gaps with respect to
experimental data is caused by a shortage in the DFT
Figure 7. Real space view of the charge redistribution ΔQ =
Q(TiO2/CNT) − (QTiO2
+ QCNT) for (a) Ti2O4 physisorbed on a hollow
site of pure CNT, (b) Ti2O4 physisorbed on an epoxy site of
functionalized CNT, (c) Ti2O4 chemisorbed on a carboxylate site of
functionalized CNT, and (d) Ti2O4 chemisorbed on a hydroxylate site
of functionalized CNT. The isosurface value is set to −20.0 and the
opacity to 0.50.
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7. estimation, mostly due to the self-correlation error of electrons
and to the difference between small clusters and bulk
matter.45,46
By comparing the C 2p bands of armchair SWCNT in Figure
8a and b, it is immediately apparent that armchair SWCNTs
with anatase adsorbed at a hollow site and a rotated hollow site
do not alter quantitatively the PDOS of C atoms. The carbon
bands of SWCNTs have no forward or backward movement of
their Fermi levels, as the titania adsorbates are nonmetallic and
nonionic species. Some additional peaks below and above the
Fermi level of the C 2p band with anatase can be seen
compared to the C 2p band with anatase adsorbed on the
rotated hollow site. We assume these peaks are characteristic of
the structural defects of SWCNTs at the adsorption region due
to C−C bond deformation.47,48
Preferential Adsorption and Direction of Growth of
TiO2 on Carbon-Based Curved Substrates. After having an
accurate understanding of the SWCNT curved surface
properties and the adsorption of titania species, a window
opens to engineer surfaces seeking desired directions of
growth.49
According to the above discussions, anatase
nanostructures showed a higher binding strength on rotated
adsorption sites with respect to normal adsorption sites if the
rotation puts the nanostructure in a normal direction with
respect to the SWCNTs. The orientation of rutile nanostruc-
tures does not affect the binding strength on armchair and
zigzag SWCNTs. Figure 9 shows the electrostatic potential
maps calculated at the ground states of titania−SWCNT
systems. Figure 9a shows the configuration for which anatase
adsorbed on SWCNT is unfavorable, especially if anastase sits
on the hollow site. This configuration makes growth on the
[001] direction unlikely, even though that direction is one of
the most favorable for anatase.49
Figure 9b shows that anatase
can then possibly grow along the [001] direction with a
maximal electrostatic potential concentrated toward that
direction. Anatase can also possibly grow along the [010] and
[0-10] directions. On the contrary, rutile adsorbed on
SWCNTs along the direction of the CNTs does not affect
the binding strength but it limits the possible direction of
growth. Figure 9c shows that rutile can possibly grow along the
[001] direction similarly to anatase, causing a maximum of
Figure 8. Partial density of states (PDOS) for anatase TiO2
physisorbed on armchair SWCNT at (a) a hollow site and (b) a
rotated hollow site. C 2p curves of both systems have been separated
from O 2p and Ti 3d curves for clarity.
Figure 9. Electrostatic potential (EP) maps for anatase and rutile species physisorbed on SWCNT. Anatase sits on (a) the hollow site of armchair
SWCNT and (b) the rotated hollow site of armchair SWCNT. Rutile sits on (c) the hollow site of armchair SWCNT and (c) the rotated hollow site
of armchair SWCNT. Note that geometrically the hollow site configurations of armchair SWCNT correspond to the rotated hollow site
configuration of zigzag SWCNT, while the rotated hollow site configurations correspond to the hollow site configurations. The isosurface value is set
to +3.0 and the opacity to 0.50.
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8. electrostatic potential located toward that direction. Rutile can
also grow along the [010] and [0-10] similarly to anatase. A 90°
rotation of anatase on similar adsorption sites, as shown by
Figure 9c, increases the binding strength by 1.05 and 0.55 eV
on armchair and zigzag SWCNT, respectively. This is
essentially due to the fact that the electrostatic potential
located at the edge of the anatase adsorbate points in a normal
direction from the nanotubes. The rotation of rutile in Figure
9d does not alter the binding strength with SWCNT but opens
a new direction of growth in the [100] and [-100] directions.
■ CONCLUSION
We have studied the adsorption of molecular and structural
TiO2 adsorbates on pure and functionalized (i.e., −COOH,
−OH, −O−) SWCNTs by density functional theory.
We concentrated our investigation on both the metal−
carbon (Ti−C) and metal−oxygen (Ti−O) interactions with a
potential two-dimensional rotation of the adsorbates that leads
to new sites, termed “rotated”. The adsorption results show that
structural TiO2 nanocompounds such as rutile (Ti2O4) and
anatase (Ti4O8) have higher binding energies to SWCNTs than
those from a simple TiO2 molecule. TixO2x (x = 2, 4)
nanostructures showed higher physical and chemical adsorption
on both armchair and zigzag SWCNTs. Anatase nanostructures
bind closer to the physical adsorption sites of SWCNTs, while
rutile nanostructures bind stronger.
Our charge redistribution maps confirmed the experimentally
found evidence that the incorporation of functionalized
SWCNTs in the TiO2 support provides more efficient electron
transfer through the film in dye-sensitized solar cells.38
The
electron transport is in fact facilitated by functional groups such
as COOH, OH, and −O− located at the edges of SWCNTs
binding to the anatase or rutile TiO2 substrates. The binding
regions have been found to have a higher electrostatic density
that provides a better electron transfer. However, experimental
studies also show that pure SWCNTs are also used as electron
transfer facilitators through TiO2 supports.36,38,50
We predict
that, at the physical adsorption binding distance, SWCNTs
provide a higher electron transfer compared to noncovalently
and interfacially bound TiO2−SWCNT, leading to improved
insight into nanocomposite photocatalytic enhancement
mechanisms.
A better understanding on the orientation of titania
nanostructures on SWCNTs has been shown by performing
electrostatic potential calculations on TiO2−SWCNT systems.
This study is of importance, as numerous experimental
researchers have studied the transport route of photogenerated
electrons in semiconducting electrodes such as TiO2.37
Both
armchair and zigzag SWCNTs were found to have an excess of
electrostatic potential located at the opposite direction of the
binding region. Further improvements in the photovoltaic
performances of the DSSCs can be achieved by predicting
better interfacial interaction between TiO2 and SWCNTs.
These results show the utility of density functional theory for
examining SWCNT−TiO2 interactions for understanding the
growth mechanisms for future experimental investigations with
this promising system for photovoltaic and photocatalytic
applications. The effect of rotations, for larger nanostructures
on the strength of the binding energy of substrate−adsorbate
systems, has been demonstrated theoretically to be of
importance. Extension of our work for experimentally
examining functionalization in pure and functionalized CNT−
titania systems is ongoing.
■ ASSOCIATED CONTENT
*S Supporting Information
Supercell parameters are given for (5,5) armchair SWCNT and
(8,0) zigzag SWCNT. Details of the organic functionalization
of SWCNT substrates are included. Additional calculation
details are provided for molecular TiO2, rutile Ti2O4, and
anatase Ti4O8. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.jpcc.5b01406.
■ AUTHOR INFORMATION
Corresponding Author
*E-mail: pcharpentier@eng.uwo.ca.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
W.A.H. acknowledges EPSRC support for the UKCP
consortium, Grant No. EP/K013610/1. S.A. and P.A.C. also
thank the Natural Sciences and Engineering Research Council
of Canada (NSERC) for funding this research and SHARC-Net
for providing the computing facilities to perform the
simulations. K.P. acknowledges the Bolyai Grant of the
Hungarian Academy of Sciences.
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